The invention relates to an improved support material especially suitable for immobilizing microorganisms in a fixed-bed or fluidized bed bioreactor, and to processes for using said support.
Support materials are generally known, e.g., from DE-OS No. 34 10 650, corresponding in part to U.S. application Ser. No. 715,314, filed Mar. 25, 1985. They make possible high space-time yields (multiplication rates per unit of volume and time) both in aerobic and in anaerobic processes of bioengineering.
In anaerobic systems, in which the attainable multiplication rates and concentrations of the biomass are considerably smaller than in aerobic systems, it is generally necessary to retain and concentrate the biomass in the reactor. As a rule, a fixed-bed circulating reactor is well suited for this purpose. In it, the support material, in which the microorganisms are immobilized, is largely stationary, and a liquid containing dissolved solids as nutrients flows by the support material so as to contact the microorganisms and effect mass transfer. Thus, the microorganisms can multiply undisturbed, and relatively high space-time yields can also be attained in anaerobic processes. But an essential drawback of this process is that the liquid must be substantially free of undissolved solids. Moreover, the bioreactor must be cleaned at determined intervals; otherwise, clogging of the fixed bed with microorganisms, reaction products or organic compounds, such as for example metal sulfides, can occur. Finally, in a fixed-bed reactor there is basically the drawback that the mass transfer takes place slowly and unevenly.
In aerobic processes, a rapid microbial growth is generally present so that, as a rule, a part of the biomass must be continually removed from the bioreactor, for example by flotation, to keep a desired concentration of the biomass in the bioreactor. Moreover, large amounts of nutrient and degradation products must be quickly transported. Therefore, resort has been made to the use of a fluidized bed bioreactor; in it, because of the constant movement of the support and nutrient solution serving as fluidized medium, a good supply of nutrient to the microorganisms and, in aerobic processes, of oxygen, is achievable. To use this expected better supply (and removal) to a larger extent, it has become known in this connection to use porous support material and thus to increase the amount of microorganisms per support body (DE No. 34 10 650). However, it has been shown that the increase of the metabolism-conversion was smaller than expected and that the support bodies easily became choked with biomass by overgrowth; the latter disadvantage, in turn, leads to an unfavorable effect on the attainable metabolism-conversion. (Patent DE-PS No. 31 05 768 is also pertinent to the invention and is discussed below.)